The present invention pertains to the field of microcellular polyurethane elastomers. More particularly, the present invention pertains to microcellular elastomers suitable for athletic midsoles prepared by the diol chain extension of an isocyanate-terminated prepolymer having an NCO group content in the range of 6-16 weight percent, prepared from a high functionality, high molecular weight polyoxyalkylene diol in the presence of water as a reactive blowing agent.
Microcellular polyurethane elastomers have become increasingly important in the athletic shoe and other markets. The midsoles of athletic shoes in particular require high resiliency and low compression set while maintaining suitable hardness. The density of the microcellular midsoles is also important, as shoes of light weight are generally desired. The method currently used by the industry to prepare microcellular midsoles and other products of similar density is to chain extend an 18% NCO content, isocyanate-terminated prepolymer with an aliphatic diol. The Shore A hardness may be adjusted within the range of Shore A 30 to Shore A 50 by varying the type and amount of chain extenders and cross-linkers.
The prior art products have several defects which can be improved upon, however. For example, midsoles and other microcellular components are normally molded to a density of from 0.35 to 0.50 g/cm3 to overcome problems with respect to shrinkage. If the microcellular foam product shrinks appreciably following demold, it will not possess the desired design dimensions. As a result, the mold must be designed, largely by trial and error, to produce a dimensionally larger part which, upon shrinking, has the desired final dimensions. It would be advantageous to produce microcellular elastomers of lower density without undue shrinkage. The microcellular foam product must also possess the necessary tensile strength, elongation, and tear strength necessary to maintain integrity not only during use, but in manufacturing processes including demolding and insertion into fabric surrounds, etc. Finally, the compression set must be low, otherwise a great deal of the cushioning will be lost after the first few periods of use.
Of the raw material components used in the preparation of microcellular elastomers, the isocyanate component of the isocyanate-terminated prepolymer is generally the most expensive. However, lowering the % NCO content of the isocyanate-terminated prepolymers used to prepare the elastomers has not been successful heretofore, as lower NCO content prepolymers produced a softer elastomer which does not provide sufficient support. Thus, the industry standard for microcellular elastomer isocyanate-terminated prepolymers has remained at 18% NCO.
The use of higher molecular weight polyoxyalkylene polyols during the preparation of the isocyanate-terminated prepolymers would allow the preparation of lower NCO content prepolymers and the use of less isocyanate relative to polyol weight. However, until recently, higher molecular weight polyols of suitable functionality have not been available. Moreover, the use of a higher molecular weight polyol should increase the soft segment content of the elastomer, which would be expected to produce an elastomer of insufficient hardness.
Polyoxypropylene diols used in prepolymer preparation have been limited to molecular weights of c.a. 2000 Da (Daltons) or less due to the rearrangement of propylene oxide to allyl alcohol during base-catalyzed oxypropylation, and the subsequent oxypropylation of this monohydric species. As a result of this phenomenon, during continued oxyalkylation, the mol percentage of monomeric oligomers steadily increases until the gain in average molecular ceases to increase further. At this point, the monofunctional oligomers may comprise 50 mol percent of the product, and the theoretical, or xe2x80x9cnominalxe2x80x9d functionality of two will be lowered to c.a. 1.6 to 1.7 or less. Thus, the preparation of high molecular weight polyoxypropylene diols having actual functionalities close to two has been impossible.
Numerous attempts have been made to lower the monol content of polyoxyalkylene polyols, but few have been successful. Use of more expensive and less commonly used basic catalysts such as cesium or rubidium hydroxides, and barium or strontium oxides or hydroxides have resulted in some improvement, as has lowering catalyst level and oxyalkylation temperatures, the latter two measures at the expense of greatly increased process time. Moreover, the improvement, as measured by the amount of unsaturation by ASTM D-2849-69, xe2x80x9cTesting Urethane Foam Polyol Raw Materials,xe2x80x9d has been marginal at best.
Use of metal naphthenates, alone or in conjunction with tertiary amine co-catalysts, has resulted in lowering the unsaturation from the normal range of 0.06-0.12 meq/g to about 0.03-0.04 meq/g. However, at the latter level, a 4000 Da polyoxypropylene diol still contains c.a. 15% monol, and has an actual average functionality of only about 1.85.
The use of double metal cyanide complex catalysts as oxyalkylation catalysts was proposed in the decade of the 1960""s and improved catalysts, such as those disclosed in U.S. Pat. No. 5,158,922 have lowered unsaturation to the range of 0.015 to 0.018 meq/g. However, the polyol products still contain from 5 to 10% monol, and commercialization has been problematic due to the cost/activity ratio of the catalyst as well as the difficulty of removing catalyst residues from the polyol product. Most recently, the ARCO Chemical Company has introduced double metal cyanide complex catalysts of much higher activity. In addition, these catalysts allow easy removal of catalyst residues from the polyol product by simple filtration. As a result, polyoxyalkylene polyols of exceptionally low unsaturation, in the range of 0.003 to 0.007 meq/g have now been commercialized. Although the exceptionally low but measurable unsaturation implies a finite monol concentration, conventional gel permeation chromatography fails to detect a low molecular weight component which would correspond to monofunctional species. The content of monol is believed to be less than 2 mol percent. Moreover, the product polyols have very low polydispersities, generally in the range of Mw/Mn=1.10, thus being virtually xe2x80x9cmonodispersexe2x80x9d.
Despite long standing efforts to lower unsaturation in polyoxyalkylene polyols, exceptionally low unsaturation polyols are not generally xe2x80x9cdrop inxe2x80x9d replacements for conventional, base-catalyzed polyols. For example, R. L. Mascioli, xe2x80x9cUrethane Applications For Novel High Molecular Weight Polyolsxe2x80x9d, 32nd Annual Polyurethane Technical/Marketing Conference, Oct. 1-4, 1989, disclosed that substitution of a 11,000 Da molecular weight triol having an unsaturation of about 0.007 meq/g for a conventional, base-catalyzed triol having a calculated molecular weight of c.a. 6000 and an unsaturation of 0.084 meq/g, resulted in a stiff and boardy foam product, whereas a softer product would be expected in view of the higher polyol molecular weight.
C. P. Smith et al., xe2x80x9cThermoplastic Polyurethane Elastomers Made From High Molecular Weight Poly-L(trademark) Polyolsxe2x80x9d, Polyurethanes World Congressxe2x80x94Sep. 24-26, pp. 313-318, disclosed one-shot polyurethane elastomers prepared from high molecular weight, high primary hydroxyl, polyoxyethylene-capped diols having measured unsaturations in the range of 0.012 to 0.016 meq/g. As expected, cast elastomers prepared from a 6600 Da molecular weight diol were softer than those prepared from a 4000 Da diol, and of similar hardness to those prepared from a conventional base-catalyzed diol, although having improved elongation and tensile strength.
In U.S. Pat. No. 4,985,491 are disclosed moisture cured polyurethane sealants containing very low NCO group content prepolymers prepared from high molecular weight triols. Sealants prepared from high molecular weight polyoxyethylene capped (10,000 Da) triols had significantly lower hardness than those prepared from 5800 Da triols. Both triols had unsaturations in the range of 0.018 to 0.022 meq/g, and monol content ranging from 5 to 19 mol percent. In U.S. Pat. No. 5,136,010 are disclosed butanediol extended cast elastomers prepared from 9% NCO isocyanate-terminated prepolymers. Elastomers prepared from higher molecular weight, low monol content diols had similar hardness to those prepared both from lower molecular weight, low monol diols as well as conventionally, base-catalyzed, high monol content diols.
It would be advantageous to be able to provide a method of preparing low density microcellular elastomers having suitable hardness while employing isocyanate-terminated prepolymers having less NCO group content than the 18% NCO industry standard. It would be further desirable to provide a method of preparing low density microcellular elastomers having improved compression set, tear strength, and resiliency. It would be yet further desirable to provide microcellular elastomers which offer little or no shrinkage upon demold.
It has now been unexpectedly discovered that polyurethane water-blown microcellular elastomers with improved properties may be prepared by the diol chain extension of an isocyanate-terminated prepolymer having a free NCO group content of between 6 weight percent and about 16 weight percent, when the prepolymer is prepared by reacting a polyoxypropylene diol having a molecular weight of from about 3000 Da to about 10,000 Da and an actual functionality greater than about 1.95, with a stoichiometric excess of a di- or polyisocyanate. The microcellular elastomers are suitable for applications such as athletic shoe midsoles, tires, athletic seating, and other applications, and most surprisingly exhibit little shrinkage.
The low density microcellular elastomers of the subject invention have densities in the range of 0.15 g/cm3 to about 0.6 g/cm3, and are prepared by a prepolymer process involving the reaction of an isocyanate component (A-side) with a polyol component (B-side). The isocyanate component comprises in substantial part an isocyanate-terminated prepolymer having an NCO group content of from 6 to about 16 weight percent, prepared by reacting a stoichiometric excess of one or more di- or polyisocyanates with one or more polyoxypropylene diols having a molecular weight of 3000 Da to 10,000 Da and a functionality greater or equal to 1.95. All molecular weights and equivalent weights herein are number average molecular and equivalent weights expressed in Da (Daltons). The polyol component comprises in substantial part one or more diol chain extenders, and optionally one or more polymer polyols or polymer-modified polyols, hereinafter termed polyol polymer dispersions. The isocyanate component and polyol component are reacted at an isocyanate index of from 70 to 130, preferably 90 to 110, and most preferably 95 to 105.
The isocyanate component is a critical aspect of the subject invention. The isocyanate component comprises in substantial part, one or more isocyanate-terminated prepolymers having an NCO group content of from 6 to about 16 weight percent, preferably 8 to about 16 weight percent, prepared by reacting a stoichiometric excess of one or more di- or polyisocyanates with one or more polyoxypropylene diols having an average molecular weight of between 3000 Da and 10,000 Da and an actual functionality of about 1.95 or more, optionally with high molecular weight, high functionality, triols, tetrols, or the like, as described hereinafter. By the term xe2x80x9cin substantial partxe2x80x9d as this term relates to the isocyanate component, is meant that greater than 70 mol percent comprises such a prepolymer. Preferably, the isocyanate-terminated prepolymers just described constitute in excess of 80 mol percent of the isocyanate component, and more preferably greater than 90 mol percent.
The isocyanate-terminated prepolymers based on high molecular weight, high functionality polyoxypropylene diols are prepared by conventional prepolymer processing techniques, e.g. by reacting a stoichiometric excess of isocyanate with the diol component neat at slightly elevated temperature under an inert gas or nitrogen pad, or with the aid of a suitable quantity of a urethane reaction-promoting catalyst. Standard methods of preparation may be found in J. H. Saunders and K. C. Frisch, Polyurethanes: Chemistry And Technology, Interscience Publishers, 1963; and the Polyurethane Handbook, Gunter Oertel, Ed., Hanser Publications, (copyright) 1985.
The isocyanate used to prepare the isocyanate-terminated prepolymers may be any di- or polyisocyanate, including but not limited to aliphatic and cycloaliphatic diisocyanates such as 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 2,4- and 2,6-methylcyclohexane diisocyanate and mixtures thereof, and 2,2xe2x80x2-, 2,4xe2x80x2-, and 4,4xe2x80x2-diisocyanatocyclohexylmethane; aromatic isocyanates such as 2,4- and 2,6-toluene diisocyanates and particularly their commercial mixtures such as 80/20 and 65/35 isomer blends, 2,2xe2x80x2-, 2,4xe2x80x2-, and 4,4xe2x80x2-diphenylmethane diisocyanate and mixtures thereof, and polyphenylene polymethylene polyisocyanates. Particularly preferred isocyanates include the toluene diisocyanates and diphenylmethane diisocyanates, particularly mixtures containing predominately 4,4xe2x80x2-diphenylmethane diisocyanate (4,4xe2x80x2-MDI).
Modified isocyanates are also useful. Modified isocyanates are prepared by reacting an isocyanate with an isocyanate reactive compound or with themselves to link two or more di- or polyisocyanate molecules. Such isocyanates are commercially available and include urethane-, urea-, biuret-, carbodiimide-, uretonimine-, uretdione-, allophanate-, and isocyanate-modified isocyanates. A most preferred modified isocyanate is carbodiimide-modified MDI.
The high molecular weight, high functionality polyoxypropylene diol has a molecular weight of from about 3000 Da to 10,000 Da and an actual functionality of about 1.95 or more. The high molecular weight, high functionality diols are prepared by the oxyalkylation of a difunctional initiator molecule with propylene oxide or a mixture of alkylene oxides containing about 80 weight percent or more of propylene oxide. Suitable difunctional initiator molecules include, but are not limited to, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, neopentyl glycol, cyclohexane-dimethanol and the like, or oxyalkylated oligomers based on such xe2x80x9cmonomericxe2x80x9d initiators having equivalent weights of from about 100 Da to 500 Da. Preferably, the initiator is propylene glycol, a polyoxypropylated oligomer thereof, or an oligomer thereof prepared by oxyalkylating with a mixture of propylene oxide and ethylene oxide.
The oxyalkylation of the initiator molecule, whether a low molecular weight xe2x80x9cmonomericxe2x80x9d initiator or oligomeric initiator, is conducted in the presence of an oxyalkylation catalyst capable of producing the high molecular weight, high functionality, polyoxypropylene diol. Any suitable oxyalkylation catalyst may be used. For example, over the lower range of high molecular weight, for example in the range of 3000 Da to 4000 Da, catalysts such as metal porphryins, metal alkyls such as diethylzinc, calcium naphthenates as disclosed in U.S. Pat. No. 4,282,387, and barium or strontium oxides or hydroxides as disclosed in U.S. Pat. Nos. 5,010,187 and 5,114,619 may be used. In these lower molecular weight ranges, prior double metal cyanide complex catalysts such as those disclosed in U.S. Pat. No. 5,158,922 are also suitable.
However, it is preferable to employ as the oxyalkylation catalyst a double metal cyanide complex catalyst capable of oxypropylating to obtain a polyoxypropylene diol having an unsaturation of 0.008 meq/g or less, preferably 0.006 meq/g or less. Examples of such catalysts are substantially amorphous double metal cyanide complex catalysts prepared by intimate mixing of metal salt and metal cyanide salt solutions as disclosed in U.S. Pat. No. 5,470,813, preferably in the presence of a low molecular weight xe2x80x9corganic complexing agentxe2x80x9d such as t-butanol, and most preferably in the presence of both an organic complexing agent and a polyether polyol complexing agent. Examples of suitable oxyalkylation catalysts are presented as Examples 1-3 herein.
The oxyalkylation of the initiator molecule(s) is performed with propylene oxide or a mixture of propylene oxide and one or more additional alkylene oxides. Alkylene oxides such as 1,2- and 2,3-butylene oxide as well as other xe2x80x9chigherxe2x80x9d alkylene oxides are suitable. Preferably, however, the oxyalkylation is conducted only with propylene oxide or a mixture of propylene oxide and ethylene oxide such that the polyoxypropylene diol contains about 80% or more by weight oxypropylene moieties. If ethylene oxide is used in conjunction with propylene oxide, it is preferably introduced along with propylene oxide to form a random polyoxyethylene-containing polyoxypropylene diol. The oxyalkylation may be begun with propylene oxide and continued with a mixture of propylene oxide and ethylene oxide to produce block random copolymers.
The molecular weight of the polyoxypropylene diol is about 3000 Da to 10,000 Da, preferably about 4000 Da to 8000 Da. The actual functionality must be greater than 1.95, and is preferably greater than 1.97. Functionalities as close to the nominal, or theoretical functionality of two as possible are preferred. The high functionality diol may be used in conjunction with a high functionality triol, tetrol, or other high functionality polyol. By xe2x80x9chigh functionalityxe2x80x9d with respect to triols, tetrols, and the like is meant that the measured functionality is within 15 percent, and more preferably within 10 percent of the nominal, or initiator, functionality.
The isocyanate component may include minor amounts of additional di- or polyisocyanates, or additional isocyanate-terminated prepolymers other than the high molecular weight, high functionality, polyoxypropylene-diol derived prepolymers. The amount of additional isocyanates and isocyanate-derived prepolymers must be less than 30 mol percent, preferably less than 20 mol percent, and more preferably less than 10 mol percent. Most preferably, the isocyanate component consists exclusively of the 6 to 16% NCO, preferably 8-16% NCO prepolymers prepared from high molecular weight, high functionality polyoxypropylene diol as heretofore described.
The polyol component comprises in substantial part one or more diol chain extenders. By xe2x80x9cin substantial partxe2x80x9d relative to the polyol component is meant greater than 60 mol percent based on isocyanate-reactive components other than water. Preferably, the diol chain extender comprises 80 mol percent or more of the isocyanate reactive component of the polyol component, more preferably 85 mol percent or higher. Suitable chain extenders include but are not limited to ethylene glycol, propylene glycol, 1,3-propanediol, 1,2- and 2,3-butylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, neopentyl glycol, 1,6-hexanediol, cyclohexane-dimethanol, and the like. Most preferred is 1,4-butanediol.
Low molecular weight cross-linkers having molecular weights below about 300 Da may also be included in the polyol component. Examples are glycerine, trimethylolpropane, pentaerythritol, diethanolamine, triethanolamine, and the like. The amount of low molecular weight cross-linker is preferably quite low, for example 5 weight percent or less based on the weight of all isocyanate reactive components other than water in the polyol component, preferably 2 weight percent or less. Preferably, no low molecular weight cross-linker is used.
The polyol component may optionally contain a minor mol percentage of an additional polyoxypropylene polyol, preferably a higher functionality polyoxyalkylene polyol, and/or a minor mol percentage of a polyol polymer dispersion. Suitable higher functionality polyoxyalkylene polyols are tri- to octa-functional, predominately polyoxypropylene polyols. Higher functionality polyols used in the polyol component are preferably those having a considerable quantity of primary hydroxyl groups, preferably about 30 mol percent primary hydroxyl groups or more, more preferably about 70 mol percent primary hydroxyl groups or more. Such polyols may be prepared conventionally, i.e. base-catalyzed, but preferably through double metal cyanide complex catalysis. When the latter is used and high primary hydroxyl content is desired, the double metal cyanide complex-catalyzed oxyalkylation with propylene oxide or mixture of propylene oxide and ethylene oxide may be terminated by addition of a basic metal catalyst such as sodium or potassium hydroxide which deactivates the double metal cyanide catalyst and allows for end-capping with ethylene oxide. The higher functionality polyol may have a hydroxyl number of from 20 to about 60, more preferably 25 to 40, and advantageously about 35. A nominal functionality of three is preferred. The polyol component may contain a minor proportion of a nominally difunctional polyoxyalkylene polyol, again preferably having a primary hydroxyl content similar to the higher functionality polyols described above.
The polyol polymer dispersions useful as optional minor portion of the polyol component comprise a dispersion of polymer particles in a base polyol which may be a polyoxyalkylene polyol similar to those described immediately above. The polymer particles may comprise vinyl polymer particles prepared by the in situ polymerization of one or more vinyl monomers, preferably acrylonitrile and styrene, in the base polyol, a xe2x80x9cpolymer polyolxe2x80x9d, or solids derived from the in situ reaction of one or more di- or polyisocyanates with an isocyanate reactive component such as water, hydrazine, or alkanolamine, for example a PIPA or PHD polyol, a xe2x80x9cpolymer-modified polyol.xe2x80x9d Mixtures of polymer polyols and polymer modified polyols may be used, as may also other polyol polymer dispersions such as those prepared by blending preformed polymer particles with a base polyol or by the in situ size reduction of polymers in a base polyol.
The microcellular elastomers are prepared by reacting the isocyanate component and polyol component in the presence of a blowing agent and generally in the presence of a urethane-promoting catalyst and foam stabilizing surfactant. Suitable catalysts include the commonly used metal catalysts, for example tin salts, and amine based catalysts which also promote reaction between water and isocyanate groups. Especially preferred catalysts include triethylenediamine, N,Nxe2x80x2, Nxe2x80x3, Nxe2x80x3-tetramethyldiethylenetriamine, SA-102, an amine-type, delayed gelation catalyst available from Air Products, and UL-1, a tin mercaptide catalyst available from the Witco Chemical Co.
Suitable surfactants include the common silicone surfactants, particularly polyether silicone surfactants, such as those available from Air Products, OSi Specialties, Inc., Wacker Chemie, and Goldschmidt A.G. A preferred surfactant is Y-10788 available from OSi Specialties, Inc.
A blowing agent is a required component. The most preferred blowing agent is water, a reactive blowing agent, used in amounts effective to provide a foam density of from about 0.15 g/cm3 to about 0.6 g/cm3. In addition to water, non-reactive physical blowing and/or frothing agents may be used. As physical blowing agents, for example, use may be mentioned of the lower alkanes and cycloalkanes, for example butane, isobutane, pentane, cyclopentane, and the like; hydrofluorocarbons and perfluorocarbons, and other halogenated hydrocarbons. Air and other gases which are essentially inert under the reaction conditions may be employed as frothing agents, or gases such as dichlorodifluoromethane or carbon dioxide may be used as liquids under pressure. Water, or water with a minor quantity of a physical blowing or frothing agent is preferred.